A fuel cell typically comprises a set of individual cells each constituted by two electrodes (anode and cathode) separated by a member acting as an electrolyte and assembled to one another in series so as to form a stack. By feeding each electrode with suitable reagent, i.e. fuel for one of the electrodes and oxidizer for the other, an electrochemical reaction is obtained that enables a potential difference to be created between the electrodes, and thus enables electricity to be produced. The stack corresponds to the core of the fuel cell, since it is within the stack that the electrochemical reaction takes place that enables electricity to be generated.
In order to feed each electrode with reagent, specific interface elements are used that are generally referred to as “bipolar plates” and that are disposed on either side of each individual cell. These bipolar plates are generally in the form of a single component placed adjacent to the anode or cathode support. In general, the fluids are distributed within the stack by two pairs of channels disposed in each face of the plate, each pair serving to deliver and return or exhaust the inert fraction of the fluid in question. Holes made through the thickness of the plates provide local feeds and exhausts for pairs of channels that extend in a sinuous configuration so as to cover the entire active surface of the individual cell. Examples of such plates are described in particular in document FR 03/12718.
Nevertheless, that type of bipolar plate is designed to deliver pure reagents. The main preoccupation is to provide a sufficient quantity of reagent to feed each distribution channel in full. The use of pure reagent leads to a high operating cost for the fuel cell and poses problems of storage for the reagents, in particular for the hydrogen.
Consequently, attempts are being made to develop fuel cells that operate with reagent fluids that are less expensive and easier to use. In general, these fluids are mixtures of gases and contain, for the fuel: a hydrogen fraction; and for the oxidizer, an oxygen fraction. These fractions lie typically in the range 20% to 100%, with the other components being for the most part nitrogen, carbon dioxide, and water vapor. For example, in most air-breathing applications, hydrogen is produced by reforming a natural gas or a hydrocarbon, corresponding to a mixture of gases typically containing 40% hydrogen, with the remaining 60% being essentially nitrogen and carbon dioxide gas. The gas mixture used as an oxidizer is generally air, i.e. 21% oxygen, 78% nitrogen, and 1% rare gases.
Nevertheless, when using such gaseous mixtures with the above-mentioned bipolar plates, the fraction of the fuel or oxidizer contained in the gaseous mixture used decreases along its path within a distribution channel, and it must not be consumed in full before the end of the channel. Consequently, with the above-described bipolar plates designed for pure reagents, it is not possible to provide a uniform distribution of the reagent species over the entire working area of the membrane, thereby degrading the overall efficiency of the fuel cell and its lifetime.